Modern diesel and gasoline internal combustion (IC) engines use significant amounts of EGR to reduce the emission of nitric oxides (commonly known as NOx). In some diesel applications the level of EGR achieved is even more important as inability to achieve the required level of EGR may cause stability problems particularly in non-conventional or special combustion modes, e.g. pre-mixed charge compression ignition (PCCI). And, certain other combustion modes, e.g. controlled auto-ignition, may have greater sensitivity to EGR concentrations also.
Particular levels of EGR may be achieved by using a closed-loop control system for the EGR valve position where the set-points of the closed-loop system are calibrated in terms of fresh airflow as seen by a mass airflow (MAF) sensor. Such set-points work as long as there is no error in the MAF sensor and also if there are no other differences in how the engine subsystems operate in comparison to the calibration. For example, any change in boost pressure, exhaust back-pressure, intake manifold temperature or other factors (e.g. ambient air temp and humidity) that affect volumetric efficiency of the engine will have an adverse effect on the EGR control system and thus the actual EGR achieved will be quite different from the calibrated value, thus causing undesirable levels of NOx emissions or instability of the combustion process.
Therefore, what is needed is a more precise EGR control.